A solder connection on a semiconductor device includes several metal containing materials, including the solder bump itself which usually comprises low melting metal alloys or metal mixtures based on tin or lead. Less obvious are the materials underneath the solder bump which bridge the conductive connection between the solder bump and the first metallization contact that is formed at the back end of line processes of a semiconductor device. These under bump materials (UBM) are also referred to as the ball limiting metallurgical (BLM) layers, since they form the foundation of the solder bump and the solder material holds only at the area where the ball limiting metals are present.
In one conventional method, a refractory laminate of sputtered BLM films is wet etched after C4 (controlled collapse chip connection) plating using the C4 as the etch mask. For lead free (Pb-free) or lead reduced (Pb-reduced) processing, the top layer of the BLM typically comprises copper or copper and an additional barrier film such as nickel or nickel alloy, the barrier layer typically being electroplated. In the case for which the top layer is copper, the copper reacts with the tin based solder material to form an intermetallic CuSn barrier layer which is important to the reliability of the Pb-free or Pb-reduced C4 bump. In the case of copper and barrier material (e.g. Ni or NiX) the barrier prevents interdiffusion of Sn and Cu, while the Cu serves as a conductive layer to enhance electroplating of Ni.
When the copper and underlying films are wet-etched (using the solder bump and/or the Ni barrier as an etch mask), there rises the problem of under bump corrosion or undercut. This wet etch undercut is variable, and has the effect of reducing the BLM footprint at the joining interface between BLM and C4. This under bump corrosion can reach dimensions of up to 10 μm of lateral lost space intended for metal contacts. This, in turn, reduces the potential integrity of bump attachment.
As semiconductor devices become smaller, a need for smaller solder connection is also required. In such case, as the C4 pitch becomes smaller, process control becomes even more critical with respect to the reliability and utility of the final C4 structure since a relatively consistent undercut per edge represents a greater threat to the integrity of the final overall C4 structure. Also, it should be understood that as semiconductor devices become smaller, a point is reached where a 10 μm undercut is not acceptable and will significantly degrade device performance.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.